Data Over Cable Service Interface Specification (DOCSIS) is a standard developed by CableLabs, which allows transparent bi-directional transfer of Internet Protocol (IP) traffic, between the cable system headend 1 and customer locations 2, over an all-coaxial or hybrid fiber/coax (HFC) cable network 3, illustrated schematically in FIG. 1.
The transmission path over the cable system is realized at the headend by a Cable Modem Termination System (CMTS) 4, and at each customer location by a Cable Modem (CM) 5. At the headend 4 (or hub), the interface to the data-over-cable system is called the Cable Modem Termination System—Network-Side Interface 6 (CMTS-NSI). At the customer locations, the interface is called the cable-modem-to-customer-premises-equipment interface 7 (CMCI). The intent is for operators to be able to transparently transfer IP traffic between the CMTS NSI interface 6 and the CMCI interface 7, including but not limited to datagrams, dynamic host configuration protocol (DHCP), internet control message protocol (ICMP), and IP Group addressing (broadcast and unicast).
The DOCSIS upstream channel uses a frequency domain/time domain multiple access (FDMA/TDMA) burst modulation format, which supports multiple symbol rates and formats (QPSK, xQAM). The modulation format includes pulse shaping for spectral efficiency, is carrier-frequency agile, and has selectable output power level. Each burst is variable in length and supports a flexible modulation, symbol rate, preamble, randomization of payload, and programmable forward error correction (FEC) encoding. The burst timing is very precise and always begins on boundaries spaced at integer multiples of 6.25 μsec.
All of the upstream transmission parameters associated with burst transmission outputs from the cable modem 5 are configurable by the CMTS 4 via media access controller (MAC) messaging. Many of the parameters are programmable on a burst-by-burst basis.
The upstream channel can support a near-continuous mode of transmission, wherein ramp-down of one burst may overlap the ramp-up of the following burst, so that the transmitted envelope is never zero. The system timing of the TDMA transmissions from the various cable modems 5 guarantees that the center of the last symbol of one burst and the center of the first symbol of the preamble of an immediately following burst are separated by at least the duration of five symbols.
An upstream modulator is part of each cable modem 5, which interfaces with the cable network 3 via coax cable and conveniently located nodes 8, which convert the RF signals to optical signals for hybrid optical/electrical networks. The upstream modulator contains the actual electrical-level modulation function and the digital signal-processing function. The digital signal-processing function provides the forward error correction (FEC), preamble prepend, symbol mapping, and other processing steps.
The CMTS 4 includes a receiver Rx for receiving the upstream channels from the fiber (or cable) network 3, and an upstream filter splitter and filter bank, which forms an upstream RF interface for the CMTS 4. The upstream splitter and filter bank separates the upstream signal into individual channels, and directs the individual channels to a demodulator Demod in the CMTS 4. The CMTS 4 accesses the various networks 1, e.g. Telco, TV, DATA, via the CMTS-NSI 6. The CMTS 4 also controls modulation of downstream channels via a CMTS downstream RF interface.
With reference to FIG. 3, the upstream channel 11 is modeled as a stream of mini-slots 12. The CMTS 4 generates the time reference for identifying the mini-slots 12, and controls access to the mini-slots 12 by the cable modems 5 using an allocation map mechanism. For example, the CMTS 4 may grant some number of contiguous slots to a cable modem 5, upon a request therefrom, for the cable modem 5 to transmit a data protocol data unit (PDU). The granted time slots may be of arbitrary length from a few tens of microseconds to a few milliseconds. The cable modem 5 must time the transmission so that the CMTS 4 receives the transmission in the time reference specified.
The CMTS 4 continuously generates allocation maps that describes the uses of the mini-slots 12 of the upstream channel 11 for a specific time interval and broadcasts the allocation maps to all cable modems 5 in MAC Management messages in a Map PDU 14 on a downstream channel 15. A given map describes how a plurality of contiguous mini-slots 12 are to be used by the CMs 5 and how the CMs 5 should access them, e.g. some mini-slots 13a as grants for particular cable modems 5 to transmit data in, other mini-slots 13b as available for contention transmission, and other mini-slots 13c as an opportunity for new cable modems to join the link. Among the time slots mapped by the CMTS is a slot called the “station maintenance” time slot (SMTS) made up of 3 to 5 mini-slots 13d. In many implementations, each cable modem receives at least one SMTS every 30 seconds. When a cable modem 5 receives an SMTS it must transmit a ranging request (RNG-REQ) message which is used by the CMTS 4 in order to determine if the cable modem 5 is operational and to maintain the link to that cable modem. If the cable modem 5 does not respond to the SMTS after 16 tries the cable modem 5 is assumed to be off.
FIG. 4 illustrates the interchange between one of the CM's 5 and the CMTS 4 when the CM 5 has data to transmit to the CMTS 4. At time t1, the CMTS 4 transmits a map PDU 14, whose effective starting time is t3, on the downstream channel 15. Within the map PDU 14 is a Request IE which will start at t5. The difference between t1 and t3 is needed to allow for:
a) downstream propagation delay (including FEC interleaving) to allow all CM's 5 to receive the Map PDU 14;
b) processing time at the CM's 5, which enables the CM's 5 to parse the Map PDU 14 and translate it into transmission opportunities; and
c) upstream propagation delay, which enables the CM's transmission of the first upstream data to begin in time to arrive at the CMTS 4 at time t3.
At t2, the CM 5 receives the map PDU 14 and scans it for request opportunities. In order to minimize request collisions, the CM 5 calculates t6 as a random offset based on the Data Backoff Start value in the most recent Map PDU 14.
At t4, the CM 5 transmits a request for as many mini-slots 12 as needed to accommodate a data PDU for the CMTS 4. Time t4 is chosen based on the ranging offset so that the request will arrive at the CMTS 4 at t6.
At t6, the CMTS 4 receives the request and schedules it for service in the next map.
At t7, the CMTS 4 transmits a map PDU 14 with details of a map, whose effective starting time is t9. Within this map, a data grant for the CM 5 will start at t11.
At t8, the CM 5 receives the map PDU 14 and scans for its data grant.
At t10, the CM 5 transmits the data PDU so that it will arrive at the CMTS 4 at t11. Time t10 is calculated from the ranging offset as in step 3.
At step 3, the request may collide with requests from other CM's 5 and be lost. The CMTS 4 does not directly detect the collision. The CM 5 determines that a collision (or other reception failure) occurred when the next map fails to include acknowledgement of the request. The CM 4 must then perform a back-off algorithm and retry.
U.S. Pat. No. 5,943,604, entitled Echo Device for Locating Upstream Ingress Noise Gaps at Cable Television Head Ends issued Aug. 24, 1999 to Chen et al, relates to a system for identifying a transmission frequency that has less noise than other available frequency bands. The Chen et al reference requires a special packet generator, rate controller, echo device, demodulator and packet checker simply to compare noise on various upstream channels.
U.S. Pat. No. 7,246,368, entitled Cable Plant Certification Procedure Using Cable Modems issued Jul. 17, 2007 to Millet et al, relates to a system for testing the power levels of various frequencies during periods of time when the cable modem is not transmitting normal data, e.g. when the SMTS indicates a cable modem is not transmitting. Unfortunately, many networks do not have enough down time to test all of the upstream channels adequately.
United States Patent Application No. 2005/0047442 published Mar. 3, 2005 in the name of Volpe et al, relates to analyzing an upstream channel, and includes identification of the cable modem of origin based on timing information derived from a predetermined database.
An object of the present invention is to overcome the shortcomings of the prior art by providing testing system, which performs a wide range of tests during normal operation of the cable modem.